Without limiting the scope of the invention, its background is described in connection with communication between surface equipment and downhole devices during hydrocarbon production, as an example. It should be noted that the principles of the present invention are applicable not only during production, but throughout the life of a wellbore including, but not limited to, during drilling, logging, testing and completing the wellbore.
Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time communication between surface equipment and downhole devices. The utilization of real time data transmission provides substantial benefits during the production of hydrocarbons from a field. For example, monitoring of downhole conditions allows for an immediate response to potential well problems including production of water or sand.
One technique used to telemeter downhole data to the surface uses the generation and propagation of electromagnetic waves. These waves are produced by inducing an axial current into, for example, the production casing. This current produces the electromagnetic waves that include an electric field and a magnetic field, which are formed at right angles to each other. The axial current impressed on the casing is modulated with data causing the electric and magnetic fields to expand and collapse thereby allowing the data to propagate and be intercepted by a receiving system. The receiving system is typically connected to the ground or sea floor where the electromagnetic data is picked up and recorded.
As with any communication system, the intensity of the electromagnetic waves is directly related to the distance of transmission. As a result, the greater the distance of transmission, the greater the loss of power and hence the weaker the received signal at the surface. Additionally, downhole electromagnetic telemetry systems must transmit the electromagnetic waves through the earth's strata. In free air, the loss is fairly constant and predictable. When transmitting through the earth's strata, however, the amount of signal received is dependent upon the skin depth (.delta.) of the media through which the electromagnetic waves travel. Skin depth is defined as the distance at which the power from a downhole signal will attenuate by a factor of 8.69 db (approximately 7 times decrease from the initial power input), and is primarily dependent upon the frequency (f) of the transmission and the conductivity (.sigma.) of the media through which the electromagnetic waves are propagating. For example, at a frequency of 10 hz, and a conductance of 1 mho/meter (1 ohm-meter), the skin depth would be 159 meters (522 feet). Therefore, for each 522 feet in a consistent 1 mho/meter media, an 8.69 db loss occurs. Skin depth may be calculated using the following equation.
Skin Depth=.delta.=1/.sqroot. (.pi.f.mu..sigma.) where:
.pi.=3.1417; PA2 f=frequency (hz); PA2 .mu.=permeability (4.pi..times.10.sup.6); and PA2 .sigma.=conductance (mhos/meter).
As should be apparent, the higher the conductance of the transmission media, the lower the frequency must be to achieve the same transmission distance. Likewise, the lower the frequency, the greater the distance of transmission with the same amount of power.
A typical electromagnetic telemetry system that transmits vertically through the earth's strata may successfully propagate through ten (10) skin depths. In the example above, for a skin depth of 522 feet, the total transmission and successful reception depth would only be 5,220 feet. It has been found, however, that in offshore applications, the boundary between the sea and the sea floor has a nonuniform and unexpected electrical discontinuity. Conventional electromagnetic systems are, therefore, unable to effectively transmit or receive the electromagnetic signals through the boundary between the sea and the sea floor. Additionally, it has been found that conventional electromagnetic systems are unable to effectively transmit the electromagnetic signals through sea water or through the boundary layer between the sea and air.
Therefore, a need has arisen for a system that is capable of telemetering real time data between the surface and downhole devices using electromagnetic waves to carry the information. A need has also arisen for an electromagnetic telemetry system that is capable of transmitting and receiving electromagnetic signals below the sea floor and relaying the information carried in the electromagnetic signals through the sea water to the surface. Further, a need has arisen for such an electromagnetic telemetry system that is capable communicating commands to specific downhole devices and receiving confirmation that the operation requested in the command has occurred.